I've heard this argument from multiple people and it always sounds stupid, to be brutally honest. Where exactly did I say I was buying a Mac purely for gaming? People buy Macs for other reasons, but there's nothing wrong at all with taking advantage of the tools for gaming that are offered for doing it on your Mac as well.
I honestly get this same dumb argument against the fact I have an Xbox, asking why not just buy a gaming PC. It's simple, because I like the Xbox and what it offers all-round, and simply put, I like being able to play games on my main computer as well when it's possible.